This invention relates to a method for making three-dimensional photonic band-gap crystals and to the crystals obtained by said method.
For several years, the art has tried to develop satisfactory methods of building photonic crystals with a full gap in three dimensions. The band-gap of a photonic crystal is a range of forbidden frequencies within which light cannot be transmitted through the crystal, and results from a regular array of dielectric material. To make a band-gap photonic material, a large contrast is needed in the dielectric constant between the range of material, such as a semiconductor or a metal, and the surrounding medium, usually air. The lattice spacing in a photonic band-gap must be comparable to the wavelength it is desired to block; the fraction of the volume occupied by the high-dielectric material must be relatively low; and the material must not be a strong absorber of the radiation.
A procedure for fabricating a photonic crystal, that has been described in the art, is to stack rods of dielectric materials in layers, each successive layer being oriented at right angles to the layer underneath. According to the art, the rods must be offset in every other row by half their period, viz. half the distance between corresponding points of successive rods of the same layer. J. C. Fleming and Shawn-Yu Lin, in xe2x80x9cThree-dimensional photonic crystal with a stop band from 1.35 to 1.95 xcexcmxe2x80x9d, Optics Letters, Vol. 24, No.1, 1999, describe the creation of 3D photonic crystals by a combination of silicon-processing techniques. By a succession of film deposits, patterning and etching, they obtain layers of one-dimensional rods with a stacking frequency that repeats itself every four layers. In each layer the rods are parallel to each other and have a fixed pitch and the orientation of the rods on alternate layers is rotated by 90 degrees.
N. Yamamoto and S. Boda, in xe2x80x9c100-nm-Scale Alignment using Laser Beam Diffraction Pattern Observation Techniques and Wafer Fusion for Realizing Three-Dimensional Photonic Crystal Structuresxe2x80x9d, J. Appl. Phys. Vol. 37, pp.3334-3338, 1998, have proposed a method of making photonic crystals, which comprises the following steps:
1xe2x80x94forming a photonic crystal layer and etching stop layer on a substrate by epitaxial growth;
2xe2x80x94forming two-dimensional structures by semiconductor microfabrication technique, such as electron beam lithography and reactive ion etching;
3xe2x80x94stacking face to face two wafers produced in step 2 and fusing them by the wafer fusion technique;
4xe2x80x94selectively and successively etching the substrate and the etch stop layer on one side of the fused wafer;
5xe2x80x94cleaving the wafer drops thus obtained into two pieces; and
6xe2x80x94repeating the steps of wafer fusion (step 3) and the selective etching (step 4) to construct a three-dimensional structure.
When stacking wafers with striped layers precise alignment is required, and the accuracy of the alignment should be less than the operating wavelength. The aforesaid authors propose an alignment method based on the observation laser beam diffraction by the striped patterns, which act as diffraction gratings.
While the method of Yamamoto and Boda is effective, it requires ultrahigh resolution lithography as well as a large number of lithographic steps. Moreover, since this method employs high temperature deposition processes, it is not compatible with organic compounds mixing, which constitute an emerging class of inexpensive, high efficiency optical materials.
It is therefore a purpose of this invention to provide a method for making three-dimensional photonic band-gap crystals which is free of the drawbacks of the prior art and particularly does not require the said ultrahigh resolution lithography.
It is another purpose of this invention to provide such a method that is more economical than those of the prior art, at least for runs up to millions of devices, and is particularly suited for customization.
It is a further purpose of this invention to provide such a method that uses materials that are transparent over a very large optical range, from the visible to the far infrared.
it is a still further purpose of this invention to provide such a method that does not require large manufacturing installations, and in particular does not require ultra-clean clean rooms.
The method of the invention is based on the use of chalcogenide glasses. Chalcogenide glasses are glasses containing chalcogen elements, viz. sulfur, selenium or tellurium. A photosensitive chalcogenide glass has the characteristic of modifying its chemical and optical properties when illuminated by a light of the appropriate wavelength, When it is placed in an etching solution, the glass is attacked by the solution, but the etching speed is not the same for the regions that have been illuminated and those that have not been illuminated. If the speed in the illuminated regions is the higher, the glass acts as a positive photoresist. If it is lower, it acts as a negative photoresist. If the ratio of the high etching speed to the low etching speed is high, the transition between the illuminated and the non-illuminated regions is sharp. Chalcogenide glasses are discussed in the chapter on Chalcogenide Glasses in xe2x80x9cGlasses and Amorphous Materials,xe2x80x9d, J. Zarzycki Ed., John Wiley and Sons, 1991.
The chalcogenide glasses that are preferred in this invention are those that are photosensitive, have a high enough refractive index, e.g. higher than 2.5, and a ratio of high etching speed to low etching speed of at least, and preferably more than, 10. Most preferred are the chalcogenide glasses chosen from AsSe, AsSeTe and AsS.
The process of the invention comprises building a three-dimensional structure by writing a series of gratings, wherein the grating material is chalcogenide glass, preferably one of the preferred chalcogenide glasses. Preferably, the writing is done using a simple interference scheme, and alternate position of chalcogenide and photoresist polymer.
More specifically, the process of the invention comprises a number of phases, each including a number of steps, as follows:
Phase Ixe2x80x94Creating a first layer, by:
axe2x80x94depositing a first chalcogenide layer or film on a substrate;
bxe2x80x94writing a first predetermined grating on the chalcogenide film, preferably using cw light;
cxe2x80x94etching the resulting illumination pattern;
dxe2x80x94covering the patterned structure with a first photoresist polymer layer, so as to obtain a thin flat layer on top of said etched chalcogenide structure, preferably using spin-coating technique;
exe2x80x94chemically polishing said first photoresist polymer layer.
Phase IIxe2x80x94Creating a second layer, by:
axe2x80x94depositing a second chalcogenide layer on top of the aforesaid polymer layer;
bxe2x80x94writing a second grating on said second chalcogenide film, perpendicular to said first grating;
c, d and exe2x80x94repeating steps c, d (using a second photoresist polymer layer), and e of Phase I.
Phase IIIxe2x80x94Creating a third layer, by:
axe2x80x94depositing a third chalcogenide layer on top of the second photoresist polymer layer;
bxe2x80x94aligning the setup and writing a third grating parallel to the first grating of the first layer obtained in Phase I, while translating in the plane of that grating perpendicularly to the grooves by half a period;
c, d and exe2x80x94repeating steps c, d (using a third photoresist polymer layer) and e of Phase I.
Phase IVxe2x80x94Creating a fourth layer by:
axe2x80x94depositing a fourth chalcogenide layer on top of the third polymer layer;
bxe2x80x94aligning the setup and writing a fourth grating parallel to said second grating of the second layer, while translating the plane of that grating perpendicular to the grooves by half a period;
c, d and exe2x80x94repeating steps c, d (using a fourth photoresist polymer layer) and e of Phase I.
The expression xe2x80x9cwriting a gratingxe2x80x9d on a photosensitive layer is to be construed as meaning illuminating the photosensitive layer according to a predetermined illumination pattern, so that when the layer is etched and the illuminated or non-illuminated portions are removed (according to whether the photoresist is positive or negative), said pattern is translated into an alternation of peaks and valleys. In this invention, the pattern consists in a plurality of straight, parallel rows, that are of equal width and equally spaced, the width of each row being generally equal to the distance between each pair of successive rows. Such a pattern is similar to that generated by a plurality of rods laid parallel to one another and at regular intervals from one another.
A block of four layers has thus been obtained. More layers should be created, in the same way, by repeating the same operations as many times as is required to achieve the desired number of layers. Generally, an increasing number of layers improves the quality of the crystal. It is estimated that sixteen layers, viz. four four-layers blocks, can be considered as infinite crystal, and therefore, in principle, repeating the aforesaid four phases four times should be sufficient.
Optionally, the whole structure thus obtained is placed in a bath of acetone or other solvent of the photoresist, whereby to remove all the photoresist, leaving a composite structure of a chalcogenide glass and holes. This step produces a crystal having the best properties and is therefore preferred. However, it is an additional manufacturing step, which has a cost, and therefore can be omitted if the crystal is intended for an application for which some reduction in the crystal""s properties, resulting from its omission, is acceptable.
The substrate, on which the first chalcogenide layer is deposited can be of any convenient material, since it supports the photonic crystal but has no part in its operation. Non-limitative examples of support materials are glass, GasAs, Silicon, and Sapphire. It thickness is not critical, and may vary, by way of non-limitative example, from a few tens of microns to a few millimeters, or even centimeters (in the case of an optical window or a long device).
The spin-coating technique is well known to persons skilled in the art. It is preferably used repeatedly in the process of the invention, each time by depositing a small quantity of photoresist on the chalcogenide glass and rotating the whole at a fast rate (for example, from 4000 to 7000 RPM) so that the photoresist is spread uniformly over the chalcogenide glass.
The steps, hereinbefore mentioned, of set-up alignment and translation in the plane of the grating perpendicularly to the grooves, will be particularly described hereinafter.